Process for making HMF from sugars with reduced byproduct formation, and improved stability HMF compositions

ABSTRACT

Disclosed is a process for making HMF or a derivative of HMF by dehydrating one or more hexose sugars in a reduced oxygen environment. In another, related aspect, a method for improving the stability and resistance to degradation of an HMF product involves adding one or more antioxidants to the HMF product.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. Ser. No.15/213,514, now allowed, which was filed Jul. 19, 2016, which was adivisional of U.S. Pat. No. 9,422,257, which was filed Sep. 1, 2015under 35 USC 371 from Patent Cooperation Treaty Application No.PCT/US2014/018186 filed Feb. 25, 2014, which in turn claimed benefit ofU.S. Ser. No. 61/782,539 filed Mar. 14, 2013.

BACKGROUND

The present invention is concerned in one aspect with processes formaking hydroxymethylfurfural from sugars, and particularly but withoutlimitation, from hexose carbohydrates such as glucose and fructose. In asecond aspect, the present invention relates to thehydroxymethylfurfural products produced by dehydration from such sugars.

Agricultural raw materials such as starch, cellulose, sucrose or inulinare inexpensive starting materials for the manufacture of hexoses, suchas glucose and fructose. Dehydrating these hexoses produces2-hydroxymethyl-5-furfuraldehyde, also known as hydroxymethylfurfural(HMF), among other products such as levulinic acid and formic acid. HMFand its related 2,5-disubstituted furanic derivatives have been viewedas having great potential for use in the field of intermediate chemicalsfrom regrowing resources. More particularly, due to its variousfunctionalities, it has been proposed that HMF could be utilized toproduce a wide range of products such as polymers, solvents,surfactants, pharmaceuticals, and plant protection agents, and HMF hasbeen reported to have antibacterial and anticorrosive properties. HMF isalso a key component, as either a starting material or intermediate, inthe synthesis of a wide variety of compounds, such as furfuryldialcohols, dialdehydes, esters, ethers, halides and carboxylic acids. Anotable example of a compound that can be prepared from HMF is2,5-furandicarboxylic acid, or FDCA, which can be prepared from HMF,ether or ester derivatives of HMF through an oxidation process, see, forexample, U.S. Pat. No. 7,317,116 and US 2009/0156841 to Sanborn et al.FDCA has been discussed as a biobased, renewable substitute forterephthalic acid, in the production of such multi-megaton polyesterpolymers as ethylene terephthalate or butylene terephthalate. FDCAesters have also recently been evaluated for replacing phthalateplasticizers for PVC, see, e.g., WO 2011/023491A1 and WO 2011/023590A1,both assigned to Evonik Oxeno GmbH, as well as R. D. Sanderson et al.,Journal of Appl. Pol. Sci. 1994, vol. 53, pp. 1785-1793.

In addition, HMF has been considered as useful for the development ofbiofuels, fuels derived from biomass as a sustainable alternative tofossil fuels. HMF has additionally been evaluated as a treatment forsickle cell anemia. In short, HMF is an important chemical compound anda method of synthesis on a large scale to produce HMF absent significantamounts of impurities, side products and remaining starting material hasbeen sought for nearly a century.

While it has correspondingly long been known that HMF can be preparedfrom sugars through dehydration, being initially prepared in 1895 fromlevulose by Dull (Chem. Ztg., 19, 216) and from sucrose by Kiermayer(Chem. Ztg., 19, 1003), chemists have differed over the years as to theprecise mechanisms by which HMF is formed from certain sugars. Asrelated very recently in Weingarten et al., “Kinetics and ReactionEngineering of Levulinic Acid Production from Aqueous GlucoseSolutions”, ChemSusChem 2012, vol. 5, pp. 1280-1290 (2012), “[o]verall,there are two schools of thought with regard to the mechanism of HMFformation from C₆ carbohydrates. One theory postulates that the reactionproceeds by way of the acyclic 1,2-enediol intermediate. The other takesinto account a fructofuranosyl cyclic intermediate in the formation ofHMF from fructose.” In relation to glucose, specifically, Weingartenreports that there are likewise two theories for how HMF is formed fromglucose: “One theory suggests that the formation of HMF from glucoseproceeds via fructose and that the near-nil presence of fructose can beattributed to its high reactivity compared to glucose. Conversely, otherauthors claim that glucose can be converted directly to HMF throughcyclization of a 3-deoxy-glucosone intermediate formed from theopen-ring form of glucose. In this respect, the relatively lowconversion of glucose to HMF is caused by its low affinity to exist inthe open-ring form due to stabilization of the glucose pyranose forms inaqueous solution.”

While there accordingly seems to be no overriding consensus as to theprecise manner in which HMF and other observed dehydration products areformed in the dehydration of hexose carbohydrates such as fructose andglucose, yet there is nevertheless a consensus that whatever mechanismsmay be at work and whatever intermediate species may be formed by suchmechanisms, a number of unwanted side products invariably are producedalong with the HMF—whether through reactions involving the intermediatespecies or involving HMF—so that an economical process to make HMF on alarge scale with good yields has not yet been realized. Complicationsarise from the rehydration of HMF, which yields by-products, such as,levulinic and formic acids. Another unwanted side reaction includes thepolymerization of HMF and/or fructose resulting in humin polymers, whichare solid waste products. Further complications may arise as a result ofsolvent selection. Water is easy to dispose of and dissolves fructose,but unfortunately, low selectivity and increased formation of polymersand hum in increases under aqueous conditions.

The realization of an economical commercial production of HMF has alsobeen hindered by HMF's comparative instability and tendency to degrade,so that purification of the HMF from the various side products and fromunconverted sugars has itself proved difficult. On long exposure totemperatures at which the desired product can be distilled, for example,HMF and impurities associated with the synthetic mixture tend to formtarry degradation products. Because of this heat instability, a fallingfilm vacuum still must be used. Even in such an apparatus, resinoussolids form on the heating surface causing a stalling in the rotor andfrequent shut down time making the operation inefficient. Prior work hasbeen performed with distillation and the addition of a non-volatilesolvent like PEG-600 to prevent the buildup of solid hum in polymers(Cope, U.S. Pat. No. 2,917,520). Unfortunately, the use of polyglycolsleads to the formation of HMF-PEG ethers.

Still other more recent efforts to deal with HMF's comparativeinstability and tendency to degrade have sought to either form morestable and easily separated HMF derivatives, for example, HMF ester andether derivatives, or to quickly remove the HMF from exposure to thoseconditions, for example, acidic conditions, tending to contribute to itsdegradation.

An example of the former approach may be found in the previously-citedUS 2009/0156841 by Sanborn et al., in which a method is provided ofproducing substantially pure HMF and HMF esters from a carbohydratesource by contacting the carbohydrate source with a solid phasecatalyst; “substantially pure” was defined as referencing a purity ofHMF of about 70% or greater, optionally about 80% or greater, or about90% or greater.

A method of producing HMF esters from a carbohydrate source and organicacids involved, in one embodiment, heating a carbohydrate startingmaterial with a solvent in a column, and continuously flowing the heatedcarbohydrate and solvent through a solid phase catalyst in the presenceof an organic acid to form a HMF ester. The solvent is removed by rotaryevaporation to provide a substantially pure HMF ester. In anotherembodiment, a carbohydrate is heated with the organic acid and a solidcatalyst in a solution to form an HMF ester. The resulting HMF ester maythen be purified by filtration, evaporation, extraction, anddistillation or any combination thereof.

An example of the latter approach may be found in WO 2009/012445 byDignan et al., wherein HMF is proposed to be made by mixing or agitatingan aqueous solution of fructose and inorganic acid catalyst with a waterimmiscible organic solvent to form an emulsion of the aqueous andorganic phases, then heating the emulsion in a flow-through reactor atelevated pressures and allowing the aqueous and organic phases to phaseseparate. HMF is present in the aqueous and organic phases in aboutequal amounts, and is removed from both, for example, by vacuumevaporation and vacuum distillation from the organic phase and bypassing the aqueous phase through an ion-exchange resin. Residualfructose stays with the aqueous phase. High fructose levels areadvocated for the initial aqueous phase, to use relatively smalleramounts of solvent in relation to the amount of fructose reacted.

In WO 2013/106136 to Sanborn et al., we described a new process formaking HMF or HMF derivatives (e.g., the ester or ether derivatives)from an aqueous hexose sugar solution in which, according to certainembodiments, the acid-catalyzed dehydration step is conducted with rapidheating of the aqueous hexose solution from an ambient to a reactiontemperature, as well as with rapid cooling of the HMF and/or HMFderivative unconverted sugar mixture prior to the separation of thefermentation-ready residual sugars product from the HMF and/or HMFderivative product. In addition, the time between when the aqueoushexose solution has been introduced into a reactor and the HMF and/orHMF ether products begin to be cooled is preferably limited.

By accepting limited per-pass conversion to HMF, the overall exposure ofthe HMF that is formed from any given aqueous hexose solution to acidic,elevated temperature conditions is limited, and preferably little to nounwanted or unusable byproducts such as humins are produced requiringwaste treatments. Separation and recovery of the products is simplifiedand levels of HMF and other hexose dehydration products known to inhibitethanol production by fermentation are reduced in the residual sugarsproduct to an extent whereby the residual sugars product can be useddirectly for ethanol fermentation if desired. Processes conducted asdescribed were characterized by very high sugar accountabilities andhigh conversion efficiencies, with very low losses of sugars beingapparent.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some of its aspects. This summary isnot an extensive overview of the invention and is intended neither toidentify key or critical elements of the invention nor to delineate itsscope. The sole purpose of this summary is to present some concepts ofthe invention in a simplified form as a prelude to the more detaileddescription that is presented later.

With this understanding, in one aspect, the invention concerns a stilldifferent approach to resolving some of the difficulties that have beenencountered in seeking to manufacture HMF on a commercial scale,especially from common hexose sugars from corn wet or dry milling orfrom the cellulosic fraction of a lignocellulosic biomass, for example,through providing a process for making HMF or a derivative of HMF bydehydrating one or more hexose sugars in a reduced oxygen environment.

In another, related aspect, the present invention concerns a method forimproving the stability and resistance to degradation of an HMF productsuch as may be produced from the acid dehydration of one or more hexosesugars, through combination of the HMF product with one or moreantioxidants, where “antioxidants” is understood to refer broadly tothose compounds and combinations of compounds which are directly orindirectly capable of limiting or even preventing, regardless of aparticular mode of action, the complex phenomena of oxidation, includingautooxidation, of organic substances of natural or synthetic origin, ofa monomeric or polymeric nature, and further concerns the improvedstability HMF compositions themselves including one or moreantioxidants. Thus, for example, “antioxidants” as used herein will beunderstood to include those materials which have been conventionallydescribed as antioxidants per se, as well as materials which have beenconventionally described or categorized differently, e.g., oxygenscavengers.

In still a further aspect, the invention concerns a method for improvingthe stability and resistance to degradation of a stored HMF product suchas may be produced from the acid dehydration of one or more hexosesugars prior to its use, comprising storing the HMF product in a reducedoxygen environment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a process according to thepresent invention in one illustrative embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The comparative instability and tendency to degrade of HMF has alreadybeen noted. The exposure of HMF to acidic, elevated temperatureconditions has been known to contribute to the degradation of HMF, butthe present invention is based upon the discovery that oxidation,including especially auto-oxidation, of HMF also plays a heretoforeunappreciated role. By conducting a dehydration of one or more hexosesugars in a reduced oxygen environment, and/or by combining HMF with oneor more antioxidants, significant improvements in the stability andresistance to degradation of the HMF can be realized.

While those skilled in the art will readily appreciate that the natureof the invention is such as permits its practical utilization with anyknown method of dehydrating one or more hexoses to form HMF or to form aderivative of HMF as desired, nevertheless for purposes of illustrationonly, one embodiment 10 of a process of the present invention is shownschematically in FIG. 1. In particular, embodiment 10 concerns amodified process otherwise according to the above-mentioned WO2013/106136 to Sanborn et al., wherein the dehydration is carried out ina reduced oxygen environment and/or wherein one or more antioxidants arecombined with the HMF and/or HMF derivatives made according to a processcarried out as described in that application, or carried out asdescribed therein but in a reduced oxygen environment for thedehydration step.

Turning now to FIG. 1, the aqueous hexose solution that is dehydrated tomake HMF or an HMF derivative can generally comprise one or more of thesix-carbon sugars (hexoses). In particular embodiments, the aqueoushexose solution can comprise one or both of the more common hexosesglucose and fructose and in certain embodiments will comprise both ofglucose and fructose. The embodiment 10 schematically shown in FIG. 1 isbased on an aqueous hexose solution including both of glucose andfructose.

In the process 10, glucose as may be derived from the hydrolysis ofstarch with acids or enzymes or from the hydrolysis of cellulosicmaterials is first enzymatically converted in step 12 through use of anisomerase to a mixture of glucose and fructose, in the form of aqueoushexose sugar solution 14. Processes for making glucose from starch andfor converting a portion of the glucose to fructose are well known, forexample, in the making of high fructose corn syrups. Alternatively, ofcourse, fructose derived from cane sugar or sugar beets, rather thanfrom an isomerization of glucose, may be combined with glucose in adesired proportion. In still another embodiment, a combination ofisomerization of glucose plus blending in of fructose from other knownsources may be employed, to provide a combination of glucose andfructose for forming an aqueous hexose sugar solution for furtherprocessing. Conveniently, the aqueous hexose sugar solution 14 cancorrespond to a current high fructose corn syrup product, for example,HFCS 42 (containing about 42 percent fructose and about 53 percentglucose), HFCS 90 (made from HFCS 42 by additional purification, about90 percent fructose and about 5 percent each of glucose and maltose) orHFCS 55 (containing about 55 percent fructose, conventionally made fromblending HFCS 42 and HFCS 90), so that existing HFCS production capacitycan be utilized to make HMF and derivative products to improve assetutilization and improve returns on capital, as HFCS demand and pricingand HMF and HMF derivative demand and pricing would indicate.

The aqueous hexose sugar solution 14 then undergoes an acid-catalyzeddehydration in step 16, to provide a mixture 18 of HMF and unconvertedsugars. Because fructose dehydrates much more readily than glucose, theproportion of glucose in the unconverted sugars of mixture 18 will behigher than in the hexose sugar solution 14. The relative amounts of HMFand of the unconverted hexose sugars in the mixture 18, and the relativeamounts of glucose and fructose in the unconverted sugars portion, canvary dependent on the manner in which the acid dehydration step 16 isconducted as well as on the composition of the aqueous hexose sugarsolution 14. In general, of course, where HMF production is to befavored over the production of ethanol from the unconverted, residualsugars, HFCS 90 will produce more HMF given the same acid dehydrationconditions than will HFCS 55, and HFCS 55 will produce more than HFCS 42(since fructose more readily dehydrates to HMF than does glucose).

In certain embodiments, as mentioned above, the acid-catalyzeddehydration step 16 is conducted with rapid heating of the aqueoushexose sugar solution 14 from an ambient temperature to the desireddehydration reaction temperature, and then with rapid cooling of theHMF/unconverted sugar mixture 18 prior to the separation of thefermentation-ready residual sugars product from the HMF product. Aswell, the time from the introduction of sugar solution 14 untilHMF/unconverted sugar mixture begins to be cooled is also limited.

By accepting limited per-pass conversion to HMF in this fashion, theoverall exposure of the HMF that is formed to acidic, elevatedtemperature conditions is correspondingly limited, so that preferablylittle to no unwanted or unusable byproducts such as hum ins areproduced requiring waste treatments. Separation and recovery of theproducts is simplified and levels of HMF and other hexose dehydrationproducts known to inhibit ethanol production by fermentation are reducedin the residual sugars product to an extent whereby the residual sugarsproduct can be used directly for ethanol fermentation if desired.

Consequently, typically the mixture 18 will comprise from 10 to 55percent molar yield of HMF, from 30 to 80 percent molar yield ofunconverted, residual sugars, and not more than 10 percent molar yieldof other materials such as furfural, levulinic acid, humins etc.Preferably, the mixture 18 will comprise from 30 to 55 percent yield ofHMF, from 40 to 70 percent yield of unconverted, residual sugars, andnot more than 5 percent yield of other materials such as furfural,levulinic acid, humins etc. More preferably, the mixture 18 willcomprise from 45 to 55 percent yield of HMF, from 25 to 40 percent yieldof unconverted, residual sugars, and not more than 5 percent yield ofother materials such as furfural, levulinic acid, humins etc.

In addition to seeking to limit the overall exposure of the HMF that isformed to acidic, elevated temperature conditions, in the illustrativeembodiment 10 of a dehydration process according to the presentinvention, the acid-catalyzed dehydration step 16 is conducted in areduced oxygen environment wherein a sub-atmospheric oxygen contentprevails. The oxygen is preferably displaced by an inert gas, forexample, nitrogen or argon. Preferably, a reduced oxygen environment isestablished within the reactor space prior to introducing the aqueoushexose sugar solution 14, or at least prior to any exposure of theaqueous hexose sugar solution 14 to an acid catalyst for carrying outthe dehydration step 16.

Returning now to FIG. 1, the HMF and unconverted, residual sugars inmixture 18 are then separated by adsorption, solvent extraction, or acombination of these in separation step 20, to yield an HMF productstream or portion 22 and a fermentation-ready sugars stream or portion24 which can optionally be supplied to an ethanol fermentation step 26for producing an ethanol product 28.

Adsorption in step 20 can be by means of any material whichpreferentially adsorbs HMF from the residual hexose sugars in themixture 18. A material which has been found to be very effective atretaining the HMF and any levulinic acid formed in the acid-catalyzeddehydration step 16 is DOWEX® OPTIPORE® V-493 macroporousstyrene-divinylbenzene resin (CAS 69011-14-9, The Dow Chemical Company,Midland, Mich.), which has been described by its manufacturer as havinga 20-50 mesh particle size, a 46 angstrom mean pore size and 1.16 mL/gpore volume, a surface area of 1100 sq. meters/g and a bulk density of680 g/liter. An ethanol wash was effective for desorbing most of theadsorbed HMF, and subsequent washing of the resin with acetone providedquantitative recovery of the HMF that was adsorbed. An alternative isAMBERLITE™ XAD™-4 polystyrene divinylbenzene polymeric adsorbent resin(CAS 37380-42-0, Rohm & Haas Company, Philadelphia, Pa.), anon-functionalized resin having a 1.08 g/mL dry density, a surface areaof 725 square meters per gram, an average pore diameter of 50 angstroms,a wet mesh size of 20-60 and a pore volume of 0.98 mL/gram. Othersuitable adsorbents can be activated carbon, zeolites, alumina, clays,non-functionalized resins (LEWATIT® AF-5, LEWATIT® S7968, LEWATIT®VPOC1064 resins, all from Lanxess AG), Amberlite® XAD-4 macroreticularcrosslinked polystryrene divinylbenzene polymer resin (CAS 37380-42-0,Rohm & Haas Company, Philadelphia, Pa.), and cation exchange resins, seeU.S. Pat. No. 7,317,116 (Sanborn) and the later U.S. Pat. No. 7,897,794(Geier and Soper). Desorption solvents may include polar organicsolvents, for example, alcohols such as ethanol, amyl alcohol, butanoland isopentyl alcohol, as well as ethyl acetate, methyl tetrahydrofuranand tetrahydrofuran.

Suitable solvents for solvent extraction include methyl ethyl ketone andespecially ethyl acetate, due to the latter's great affinity for HMF andlevulinic acid, low boiling point (77 deg. C.) and ease of separationfrom water. As demonstrated in certain of the examples of the WO2013/106136 application, virtually complete recovery of the sugars andof the HMF from mixture 18 can be accomplished through a series of ethylacetate extractions. Additionally, while the residual sugars recoveredby other means were still suitable for being directly processed toethanol in the subsequent ethanol fermentation step 26, those recoveredfollowing the quantitative extraction with ethyl acetate were observedto be significantly less inhibitory even under non-optimal conditions. Avariety of other solvents have been suggested or used in the literaturerelated to HMF and HMF derivative synthesis and recovery in biphasicsystems, and these may be appropriate for use in the context of thepresent invention. Examples of other useful solvents are butanol,isoamyl alcohol, methyl ethyl ketone, methyl isobutyl ketone, diethylether, cyclopentyl dimethyl ether, methyl tetrahydrofuran, and methylbutyl ether.

Ethanol fermentation step 26 can encompass any known process whereby ahexose sugars feed of the type represented by fermentation-ready sugarsstream or portion 24 may be converted to one or more products inclusiveof ethanol, at least in some part by fermentation means. Both aerobicand anaerobic processes are thus contemplated, using any of the varietyof yeasts (e.g., kluyveromyces lactis, kluyveromyces lipolytica,saccharomyces cerevisiae, s. uvarum, s. monacensis, s. pastorianus, s.bayanus, s. ellipsoidues, candida shehata, c. melibiosica, c.intermedia) or any of the variety of bacteria (e.g., clostridiumsporogenes, c. indolis, c. sphenoides, c. sordelli, candida bracarensis,candida dubliniensis, zymomonas mobilis, z. pomaceas) that haveethanol-producing capability from the fermentation-ready sugars streamor portion 24 under aerobic or anaerobic conditions and otherappropriate conditions. The particular yeasts (or bacteria) used andother particulars of the fermentations employing these various yeasts(or bacteria) are a matter for routine selection by those skilled in thefermentation art, though the examples below demonstrate thefunctionality of one common anaerobic yeast strain, saccharomycescerevisiae. Given that the sugars stream or portion 24 derives from aprocess for making the acid dehydration product HMF, a yeast or bacteriathat has been demonstrated for use particularly with sugars derived froma lignocellulosic biomass through acid-hydrolyzing the biomass and/or acellulosic fraction from biomass may be preferred. For example, theaerobic bacterium corynebacterium glutamicum R was evaluated in Sakai etal., “Effect of Lignocellulose-Derived Inhibitors on Growth of andEthanol Production by Growth-Arrested Corynebacterium glutamicum R”,Applied and Environmental Biology, vol. 73, no. 7, pp 2349-2353 (April2007), as an alternative to detoxification measures against organicacids, furans and phenols byproducts from the dilute acid pretreatmentof biomass, and found promising.

While the amounts of HMF (and/or HMF ethers, as the case may be) and ofunconverted, residual sugars may vary somewhat, preferably in allembodiments a high degree of sugar accountability is achieved, where“sugar accountability” is understood to refer to the percentage ofsugars input to the acid dehydration step 16 that can be accounted forin adding the molar yields of identifiable products in the mixture18—essentially adding the molar yields of HMF (and/or of HMF ethers),levulinic acid, furfural and residual, unconverted sugars. Preferably, amodified process 10 according to the present invention is characterizedby a total sugar accountability of at least 70 percent, more preferablyat least 80 percent and most preferably at least 90 percent.

The fermentation-ready sugars stream or portion 24 can, in whole or inpart, also be used for other purposes beyond the production of ethanol.For example, sugars in stream or portion 24 can be recycled to thebeginning of the acid dehydration step 16 for producing additional HMFor HMF ethers. The hexose sugars represented by stream or portion 24 canalso be hydrogenated to sugar alcohols for producing other biobasedfuels and fuel additives (other than or in addition to ethanol), see,for example, U.S. Pat. No. 7,678,950 to Yao et al. The sugars in streamor portion 24 can be fermented to produce lysine or lactic acidaccording to known methods, or used for making another dehydrationproduct such as levulinic acid. Still other uses will be evident tothose skilled in the art, given the character of the sugars stream orportion 24 provided by the described process.

A number of prospective uses of HMF product stream or portion 22 havealready been mentioned, but one important contemplated use would be inthe manufacture of 2,5-furandicarboxylic acid (FDCA) using a Mid-Centurytype Co/Mn/Br oxidation catalyst under oxidation conditions, asdescribed in United States Pat. Application Publication No. US2009/1056841 to Sanborn et al. and in Patent Cooperation TreatyApplication Ser. No. PCT/US12/52641, filed Aug. 28, 2012 for “Processfor Producing Both Biobased Succinic Acid and 2,5-FurandicarboxylicAcid”, now published as WO 2013/033081. Where the HMF product stream orportion 22 is not directly used in a transformative process or otherwisemay be exposed to an oxygen source in use such that an undesirabledegradation of the HMF (or of an HMF derivative which is susceptible ofdegrading, albeit to a lesser extent) through autooxidation isforeseeable (processes for carrying out an oxidation of the HMF productstream or portion 22 or of some portion thereof to produce, e.g., FDCA,being examples of transformative processes involving the purposefulinteraction of oxygen with HMF), preferably one or more antioxidants arecombined with the HMF product stream or portion 22 or with some portionthereof that is foreseeably exposed to an oxygen source and at risk ofdegrading through autooxidation.

While the examples below utilize butylated hydroxyanisole (BHA) as theantioxidant additive, those skilled in the art will appreciate that anumber of materials are well-known and used as antioxidants or as oxygenscavengers in other contexts of use, and it will be well within thecapabilities of those skilled in the art to select materials (orcombinations of materials) other than BHA which could be used in theprocess of the present invention, and to determine the amounts needed ofsuch materials to improve the stability and resistance to degradation ofHMF or of an HMF derivative that is otherwise susceptible to degradingthrough autooxidation. Given the substantial interest, as mentionedpreviously, in FDCA and FDCA esters for various polymer applications,those materials which have previously been found well-suited for use asantioxidants in polymer compositions are expected to find wider usecommercially in the practice of the present invention. Various suchmaterials may be found in, for example, chapter 1, entitled“Antioxidants”, of the Plastics Additive Handbook, 5th ed., Carl HanserVerlag, Munich, Germany (2001).

As previously indicated, the acid dehydration step 16 is preferablyconducted in a manner to limit per-pass conversion to HMF and theexposure of the HMF that is formed to acidic, elevated temperatureconditions. Rapid heating of the hexose sugar solution 14, as well asrapid cooling of the HMF/unconverted sugar mixture produced from theacid dehydration step 16, are desirable for accomplishing theseobjectives for a given amount of hexose sugar solution 14. Further, oncethe aqueous hexose solution 14 has reached the desired reactiontemperature range, the extent to which the aqueous hexose solutionremains subject to the acidic, elevated temperature conditions ispreferably also limited. While optimal conditions will vary somewhatfrom one embodiment to the next, for example, in processing HFCS 42versus HFCS 55 versus HFCS 90 as shown clearly in the WO 708application, in general terms for a concentrated sulfuric acid contentof about 0.5 percent by weight based on the mass of hexose sugars in thesugar solution 14 (or the equivalent acid strength, for other acidcatalysts), a reaction temperature of from 175 degrees Celsius to 205degrees Celsius, a dry solids loading of sugars in the range of from 10to 50 percent, a final dry solids concentration of from 10 to 25percent, and an average residence or reaction time of from 2 to 10minutes appear to be advantageous. “Average residence or reaction time”or similar terminology as used herein refers to the time elapsed fromthe introduction of the sugar solution 14 into a reactor until coolingof the mixture 18 is commenced.

As a general matter, of course, it would be preferable to process sugarsolutions 14 having a greater loading of the hexose sugars rather than alesser loading, though some trade-offs were observed in terms of overallsugars accountability and in other respects, and these would need to beconsidered in determining the optimum conditions to be observed for agiven feedstock. Similarly, milder reaction conditions generally providelesser conversion, but enable increased sugars accountability.

For the particular example of a 40 percent dry solids loading HFCS 42feed providing up to a 20 percent final dry solids concentration, usinga shorter reaction time and a temperature toward the higher end seempreferable, for example, 5 minutes at 200 degrees Celsius. For HFCS 90,given the same acid starting concentration, the reaction temperature canbe in the range of from 185 degrees to 205 degrees Celsius, the drysolids loading of hexose sugars in the sugar solution 14 can be from 30to 50 percent and provide an 8 to 15 percent final dry solidsconcentration, and a reaction time can be from 5 to 10 minutes.

As an illustration of the considerations involved in processing onefeedstock versus another, for HFCS 90 in contrast to HFCS 42, a finaldry solids concentration of 20 percent could not be processed with thesame overall sugars accountability, and a lower final dry solidsconcentration was indicated as preferable. For a final dry solidsconcentration of 10 percent, a reaction temperature of 185 degreesCelsius and a reaction time of 10 minutes were observed to providefavorable results. Favored conditions for the recovered sugars in streamor portion 24, it should be noted, may differ from those contemplatedfor freshly-supplied sugars in sugar solution 14 where recycle iscontemplated for making additional HMF product.

In any event, the heating to the desired reaction temperature ispreferably accomplished in not more than 15 minutes, preferably isaccomplished in 11 minutes of less, more preferably in not more than 8minutes and still more preferably is accomplished in not more than fiveminutes. As demonstrated by the examples of the WO 2013/106136application, rapid feeding of a quantity of ambient hexose sugarsolution to a hot aqueous acid matrix (in two minutes) gave consistentimprovements in one or more of HMF selectivity, yield and overall sugaraccountability compared to less rapid feeding, even given the sameelapsed time between when the quantity of hexose sugar solution wasfully introduced and when cooling was initiated. Rapid cooling from thereaction temperature to 50 degrees Celsius and lower is preferablyaccomplished in not more than 5 minutes, especially 3 minutes or less.

More particularly, in a batch reactor combining the sugar solution 14and the acid catalyst in a hot reactor already close to or at thedesired reaction temperature provides improved results as compared towhere the sugar solution 14 and acid catalyst are added to a reactor andthen heated gradually together to the desired reaction temperature.

In regard to continuous processes, one suitable means for rapidlyheating the sugar solution 14 and the acid catalyst would be directsteam injection. A commercially-available, in-line direct steaminjection device, the Hydro-Thermal Hydroheater™ from Hydro-ThermalCorporation, 400 Pilot Court, Waukesha, Wis., injects sonic velocitysteam into a thin layer of a liquid (such as the sugar solution 14)flowing from an inlet pipe through a series of gaps. Steam flow isadjusted precisely through a variable area nozzle to an extent wherebyoutlet fluid temperatures are claimed to be controllable within 0.5degrees Fahrenheit over a large liquid turndown ratio. Turbulent mixingtakes place in a specifically designed combining tube, with anadjustable degree of shear responsive to adjustments of the steam flowand the liquid flow through (or pressure drop across) the series ofgaps. Devices of this general character are described in, for example,U.S. Pat. No. 5,622,655; U.S. Pat. No. 5,842,497; U.S. Pat. No.6,082,712; and U.S. Pat. No. 7,152,851.

In the examples reported in WO 2013/106136 using such a device, thehighest HMF yield and sugar accountability from HFCS 42 syrup included asystem of sulfuric acid (0.5% by wt of sugars), an initial dry solidsconcentration of 20% and rapid heating of the reaction mixture by directsteam injection with a system back pressure of 1.48 MPa, gauge to 1.52MPa, gauge (215-220 psig), a steam pressure of 1.9 MPa, gauge (275psig), a time of 5-6 minutes at the reaction temperatures provided bythe direct steam injection and rapid cooling of the product mixturebefore pressure relief. The reaction control set point, as monitored bythe temperature control element, was 200 degrees Celsius and the maximumtemperature achieved at the end of the resting tube was 166 degreesCelsius. HMF was obtained with these conditions in up to 20% molar yieldwith greater than 90% total sugar accountability. There was virtually novisible production of insoluble hum ins.

For HFCS 90 syrup processed in the same apparatus, the highest HMF yieldand sugar accountability included a system of sulfuric acid (0.5% by wtof sugars) an initial dry solids concentration of 10% and rapid heatingof the reaction mixture by direct steam injection with a system backpressure of 1 MPa, gauge (150 psig), a steam pressure of 1.4 MPa, gauge(200 psig), a time of 11 minutes at the reaction temperatures providedby the direct steam injection and rapid cooling of the product mixturebefore pressure relief. The reaction control set point was 185 degreesC. and the maximum temperature achieved at the end of the resting tubewas 179 degrees C. HMF was obtained from HFCS 90 with these conditionsup to 31% molar yield with greater than 95% total sugar accountability.There was again virtually no visible production of insoluble hum ins.

Rapid cooling of the mixture 18 can be accomplished by various means.For example, while a brazed plate heat exchanger was used in at leastcertain of the examples below prior to a pressure reduction, other typesof exchangers could be used. Other options will be evident to those ofroutine skill in the art

It will be appreciated that the acid-catalyzed dehydration step 16 canbe conducted in a batchwise, semi-batch or continuous mode. A variety ofacid catalysts have been described previously for the dehydration ofhexose-containing materials to HMF, including both homogeneous andheterogeneous, solid acid catalysts. Solid acid catalysts would bepreferred given they are more readily separated and recovered for reuse,but selecting a catalyst that will maintain a satisfactory activity andstability in the presence of water and at the temperatures required forcarrying out the dehydration step 16 can be problematic. Sulfuric acidwas used in the examples of the WO 2013/106136 application and is usedin the examples below, and provided good yields and excellent sugaraccountabilities.

The present invention is illustrated by the following examples:

Example 1 and Comparative Example 1

Crystalline fructose (5 g) was dissolved in 7 mL of water, and chargedto a two neck 25 mL round bottom flask. The flask was equipped with atube to sparge gas (either dry air (Comp. Ex. 1) or nitrogen (Ex. 1))through the solution and with a reflux condenser. The gas (dry air ornitrogen) was sparged through the solution for five minutes, then onedrop of concentrated sulfuric acid was added to the solution. The flaskwas then closed with rubber septa, inserting a 16 gauge needle to allowsparge gas to escape, and refluxed for 7 hours. After 7 hours' reactiontime, the contents from both of the dry air-sparged and nitrogen-spargedruns were amber in color, but from proton NMR the reactor contents ofthe nitrogen-sparged run were substantially free of levulinic and formicacids (Table 1), while the conventional dry air-sparged reactor contentsshowed significant amounts of both.

TABLE 1 Fructose HMF Levulinic Acid Formic Acid Sparge Gas Conversion(%) (%) (%) (%) Nitrogen 3.17 2.99 ND 0.18 Dry air 5.8 4.91 0.93 1.57

Examples 2-5 with Comparative Examples 2-5

To a vial containing 500 mg of melted hydroxymethylfurfural was 1000 ppmby weight of butylated hydroxyanisole (BHA) (Ex. 2). The mixture wasvortex stirred, then placed in an oven set to 85 degrees Celsius. Forcomparison, a vial containing 500 mg of melted hydroxymethylfurfural butno BHA (Comp. Ex. 2) was placed in the oven alongside the first vial.

A second set of samples—Example 3 and Comparative Example 3—wereprepared by combining 850 mg of HMF with 150 mg of water; to one of thesamples was added 1000 ppm equivalent of BHA, while nothing was added tothe second. Both samples were vigorously stirred and placed in an 85deg. C. oven.

Four samples of 10 percent by weight of HMF in water were then preparedby combining 100 mg of HMF with 900 mg of water. To one of the samples(Ex. 4) was added 1000 ppm equivalent of BHA. For a second sample (Ex.5), the air was purged by bubbling argon through the solution and thevial was sealed to preserve the HMF under an argon atmosphere. For thethird sample (Comp. Ex. 4), 10% formic acid by mass of HMF was spikedinto the vial. The fourth sample (Comp. Ex. 5) was not modified at all.All four samples were again placed in the 85 degrees Celsius oven aftervigorous stirring.

The compositions of the various vials were analyzed after 1 week andthen again after 2 weeks (with the exception of the argon-spargedsample), with the results shown in Tables 2 and 3, respectively:

TABLE 2 One Week HMF Formic Levulinic Example Description (wt. %) (wt.%) (wt. %) Ex. 2 HMF w/BHA 87.26 0.12 0.32 Comp. Ex. 2 HMF 82.27 0.350.38 Ex. 3 85% HMF 81.86 0.10 0.04 w/BHA Comp. Ex. 3 85% HMF 79.51 0.280.43 Ex. 4 10% HMF 8.41 0.15 0.12 w/BHA Comp. Ex. 4 10% HMF 8.49 0.240.30 w/formic Ex. 5 10% HMF w/Ar 9.17 0.12 0.04 Comp. Ex. 5 10% HMF 8.470.15 0.13

TABLE 3 Two Weeks Example Description HMF (wt. %) Formic (g/L) Levulinic(g/L) Ex. 2 HMF w/BHA 85.61 2.82 2.25 Comp. Ex. 2 HMF 72.74 5.63 4.63Ex. 3 85% HMF 78.98 1.72 0.38 w/BHA Comp. Ex. 3 85% HMF 54.99 2.81 2.26Ex. 4 10% HMF 6.66 3.24 2.64 w/BHA Comp. Ex. 4 10% HMF 6.77 2.67 2.16w/formic Comp. Ex. 5 10% HMF 6.90 3.07 2.46

Examples 6 and 7

These examples were performed to assess whether the antioxidant BHAwould also effectively stabilize ester and ether derivatives of HMF, inaddition to HMF.

Ester Derivative:

For the ester example, 5-acetoxymethylfurfural (AcHMF) was purchasedcommercially (from Aldrich) and recrystallized from an n-hexane/methyltert-butyl ether mixture to improve its purity. To a vial containing 500ppm equivalent of BHA, 900 mg of AcHMF was added. A second samplecontaining 700 mg of AcHMF was prepared without BHA for comparison. Theheadspace was purged with argon and the AcHMF was melted and mixed. Thesamples were re-exposed to an air atmosphere and placed in an 85° C.dark oven, and analyzed after one week and again after two weeks.

The stabilizer clearly does not have an adverse effect, but at leastunder the conditions and in the timeframes tested, neither was there anyappreciable degradation so that under the recited conditions the resultswere inconclusive.

TABLE 4 Week 1 Week 2 Description (wt %) (wt %) AcHMF - Blank 94.5794.59 AcHMF - 500 ppm BHA 94.13 94.34

The HMF and AcHMF for the ester example were analyzed byultra-performance liquid chromatography (UPLC), using a Waters AcquityH-Class UPLC apparatus with TUV detector—Monitor at 280 nm, and thefollowing additional analysis details:

Column: Waters BEH C18 2.1×50 mm, 1.7 □m

Temperature: 50° C.

Flow rate: 0.5 mL/min

Purge solvent: 10% Acetonitrile

Wash solvent: 50% Acetonitrile

Solvent C: 50% Acetonitrile

Solvent D: Water

Gradient:

Time (min) % C % D initial 10 90 0.60 46 54 0.80 99 1 0.96 99 1 0.97 1090 2.50 10 90

Injection volume: 0.5 uL

Run time: 2.5 min

Ether Derivative:

For the ether derivative of HMF, 5-butoxymethylfurfural (BMF) wasrecrystallized from n-hexane until no more butyl levulinate wasobservable by NMR. BMF (700 mg) was added to a vial containing 500 ppmequivalent of BHA followed by thorough mixing. For comparison, 300 mg ofBMF was placed in a vial containing no BHA. The vials were kept in adark 85° C. oven and sampled for analysis after 1 and 2 week intervals.

The test results were as shown in Table 5:

TABLE 5 Week 1 Week 2 Description (GC % area) (GC % area) BMF - Blank98.079 96.546 BMF - 500 ppm BHA 99.327 98.773

The BMF was analyzed by gas chromatography. The sample was diluted to aconcentration of 1 mg/mL with acetonitrile and the GC area percent wasmeasured as reported in Table 5. The starting material was >99.9% by GCarea. While not all decomposition products were identified and indeedwhile some degradation products may not have been detected, neverthelessthe results demonstrate that BHA was helpful for stabilizing the etherderivative BMF.

Particular details of the analytical method were as follows:

Instrument: Agilent 7890 GC with 7693 autosampler

Column: DB-5 UI 60 m×250 □m×0.25 □m

Carrier gas: H₂

Flow rate: 1 mL/min (constant)

Injector temperature: 200° C.

Split: 50:1

Detector: 340° C.

Temperature program: Initial: 50° C.

Ramp 1: 5°/min to 180° C.

Ramp 2: 20° C./min to 200° C. hold 1 min

What is claimed is:
 1. A method for improving the degradation resistanceof stored hydroxymethylfurfural prior to use thereof, comprising storingthe hydroxymethylfurfural in a reduced oxygen environment.